From 13a21a61df57dbc90ac0e15d538dd924074afe29 Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <brad@danga.com>
Date: Sun, 8 Sep 2013 16:35:56 -0700
Subject: [PATCH] replica: allow optional readBackends config option. Default
 is same as write backends.

Change-Id: I8aae98f76449c41d937469cf91d6788c348807ed
---
 pkg/blobserver/replica/replica.go | 41 ++++++++++++++++++++-----------
 1 file changed, 27 insertions(+), 14 deletions(-)

diff --git a/pkg/blobserver/replica/replica.go b/pkg/blobserver/replica/replica.go
index 1d840eceb..63d7b9dc0 100644
--- a/pkg/blobserver/replica/replica.go
+++ b/pkg/blobserver/replica/replica.go
@@ -50,9 +50,14 @@ import (
 const buffered = 8
 
 type replicaStorage struct {
+	// Replicas for writing:
 	replicaPrefixes []string
 	replicas        []blobserver.Storage
 
+	// Replicas for reading:
+	readPrefixes []string
+	readReplicas []blobserver.Storage
+
 	// Minimum number of writes that must succeed before
 	// acknowledging success to the client.
 	minWritesForSuccess int
@@ -63,6 +68,7 @@ type replicaStorage struct {
 func newFromConfig(ld blobserver.Loader, config jsonconfig.Obj) (storage blobserver.Storage, err error) {
 	sto := &replicaStorage{
 		replicaPrefixes: config.RequiredList("backends"),
+		readPrefixes:    config.OptionalList("readBackends"),
 	}
 	nReplicas := len(sto.replicaPrefixes)
 	sto.minWritesForSuccess = config.OptionalInt("minWritesForSuccess", nReplicas)
@@ -75,19 +81,31 @@ func newFromConfig(ld blobserver.Loader, config jsonconfig.Obj) (storage blobser
 	if sto.minWritesForSuccess == 0 {
 		sto.minWritesForSuccess = nReplicas
 	}
-	sto.replicas = make([]blobserver.Storage, nReplicas)
-	for i, prefix := range sto.replicaPrefixes {
-		replicaSto, err := ld.GetStorage(prefix)
+	// readPrefixes defaults to the write prefixes.
+	if len(sto.readPrefixes) == 0 {
+		sto.readPrefixes = sto.replicaPrefixes
+	}
+
+	for _, prefix := range sto.replicaPrefixes {
+		s, err := ld.GetStorage(prefix)
 		if err != nil {
 			return nil, err
 		}
-		sto.replicas[i] = replicaSto
+		sto.replicas = append(sto.replicas, s)
+	}
+	for _, prefix := range sto.readPrefixes {
+		s, err := ld.GetStorage(prefix)
+		if err != nil {
+			return nil, err
+		}
+		sto.readReplicas = append(sto.readReplicas, s)
 	}
 	return sto, nil
 }
 
 func (sto *replicaStorage) FetchStreaming(b blob.Ref) (file io.ReadCloser, size int64, err error) {
-	for _, replica := range sto.replicas {
+	// TODO: race these? first to respond?
+	for _, replica := range sto.readReplicas {
 		file, size, err = replica.FetchStreaming(b)
 		if err == nil {
 			return
@@ -96,8 +114,7 @@ func (sto *replicaStorage) FetchStreaming(b blob.Ref) (file io.ReadCloser, size
 	return
 }
 
-// StatBlobs stats all replicas.
-// See TODO on EnumerateBlobs.
+// StatBlobs stats all read replicas.
 func (sto *replicaStorage) StatBlobs(dest chan<- blob.SizedRef, blobs []blob.Ref) error {
 	need := make(map[blob.Ref]bool)
 	for _, br := range blobs {
@@ -122,12 +139,12 @@ func (sto *replicaStorage) StatBlobs(dest chan<- blob.SizedRef, blobs []blob.Ref
 		errc <- s.StatBlobs(ch, blobs)
 	}
 
-	for _, replica := range sto.replicas {
+	for _, replica := range sto.readReplicas {
 		go statReplica(replica)
 	}
 
 	var retErr error
-	for _ = range sto.replicas {
+	for _ = range sto.readReplicas {
 		if err := <-errc; err != nil {
 			retErr = err
 		}
@@ -231,11 +248,7 @@ func (sto *replicaStorage) RemoveBlobs(blobs []blob.Ref) error {
 }
 
 func (sto *replicaStorage) EnumerateBlobs(dest chan<- blob.SizedRef, after string, limit int) error {
-	// TODO: option to enumerate from one or from all merged.  for
-	// now we'll just do all, even though it's kinda a waste.  at
-	// least then we don't miss anything if a certain node is
-	// missing some blobs temporarily
-	return blobserver.MergedEnumerate(dest, sto.replicas, after, limit)
+	return blobserver.MergedEnumerate(dest, sto.readReplicas, after, limit)
 }
 
 func init() {